1. Field of the Invention
The present invention relates to a gas discharge display device, a plasma addressed liquid crystal display device, and a method for producing the same, and more particularly to a gas discharge display device and a plasma addressed liquid crystal display device having a particular discharge electrode, and a method for producing the same.
2. Description of the Background Art
Plasma addressed liquid crystal display devices (PALCs) have been developed aiming to realize large-sized thin flat displays. A PALC is a liquid crystal display device having a structure in which a liquid crystal cell and a plasma cell are layered together via a dielectric layer therebetween, in which picture elements are switched by using plasma channels. A PALC can be made in a large size and produced at a low cost as compared to a liquid crystal display device using TFTs (Thin Film Transistors).
A plasma cell includes a plasma cell substrate and a dielectric layer, with a plurality of partition walls being arranged therebetween in a stripe pattern. Note that the dielectric layer also functions as a part of the liquid crystal cell. A plasma channel is defined as a space sealed by adjacent partition walls, the plasma cell substrate and the dielectric layer, and the plasma channel is filled with a discharge gas capable of being ionized through discharge. Each of the plasma channels has discharge electrodes (an anode and a cathode) formed on the plasma cell substrate, and the discharge gas is ionized into a plasma state by applying a voltage to the discharge electrodes. This phenomenon is called xe2x80x9cplasma dischargexe2x80x9d.
A liquid crystal cell includes a liquid crystal cell substrate and the dielectric layer, with a liquid crystal layer being interposed therebetween. On the liquid crystal layer side of the liquid crystal cell substrate, a plurality of signal electrodes in a parallel stripe pattern are formed so as to cross the plasma channels. Moreover, the liquid crystal cell substrate includes, on the liquid crystal layer side, colored layers provided so as to correspond to the signal electrodes. The colored layers are typically red, green and blue layers.
In a PALC, each region at an intersection between a signal electrode and a plasma channel defines a picture element region. The liquid crystal layer in each picture element region changes its orientation according to the voltage applied between the signal electrode and the plasma channel, whereby the amount of light passing through the picture element region changes. An image signal is applied through the liquid crystal layer in each of the picture element regions arranged in a matrix pattern, so as to control the amount of light passing through the picture element region, thereby displaying an image. In the present specification, the minimum unit of display is referred to as a xe2x80x9cpicture elementxe2x80x9d, and each region of the liquid crystal display device corresponding to a xe2x80x9cpicture elementxe2x80x9d is referred to as a xe2x80x9cpicture element regionxe2x80x9d.
A PALC operates as follows, for example, with the plasma channel being the row scanning unit and the signal electrode being the column driving unit.
A line sequential scanning operation is performed by successively and selectively turning the plasma channels into a plasma state by rows. In synchronism with this, a driving voltage is applied to each of the signal electrodes forming the column driving unit. Since a plasma channel selectively turned into a plasma state is filled with an ionized discharge gas, the potential of the plasma channel turned into a plasma state, except for the vicinity of the cathode, is substantially equal to the potential of the anode. Therefore, an amount of charge according to the difference between the potential of the plasma channel and the potential of the driving voltage is induced/stored in the bottom surface of the dielectric layer (the surface on the plasma channel side; hereinafter referred to as the xe2x80x9cdielectric layer bottom surfacexe2x80x9d) located between the plasma channel turned into a plasma state and the signal electrode opposing the plasma channel. At this time, the liquid crystal layer in the picture element region defined by a region where the plasma channel turned into a plasma state and the signal electrode to which the driving voltage is applied intersect each other changes its orientation according to a voltage obtained by capacitance division of the voltage applied to the plasma channel and the signal electrode between the dielectric layer and the liquid crystal layer.
Then, when the plasma channel is de-selected (when the plasma discharge is stopped), the inside of the plasma channel is insulated, and the state where the charge is stored in the dielectric layer bottom surface is maintained until the plasma channel is selected again to be turned into a plasma state. In other words, the potential difference (voltage) between the dielectric layer bottom surface and the signal electrode is sampled and held by the capacitance formed by the dielectric layer bottom surface, the dielectric layer, the liquid crystal layer and the signal electrode. As a result, while the inside of the plasma channel is insulated, the orientation of the liquid crystal layer in the picture element region is maintained by the sampled and held voltage.
As described above, the plasma channel functions as a switching element for controlling the electrical connection/disconnection between the dielectric layer bottom surface and the anode. Moreover, the dielectric layer bottom surface also functions as a virtual electrode. Of course, the rows and columns may be reversed, in which case the anode of the plasma channel is used as the driving unit by applying a driving voltage thereto, and the signal electrode is used as the scanning unit by applying a scanning voltage thereto.
The plasma discharge occurring in a plasma channel is initiated as follows. When a voltage is applied between an anode and a cathode, electrons emitted from the cathode are accelerated by an electric field between the anode and the cathode to collide with molecules of the discharge gas filled in the plasma channel while traveling toward the anode. As a result, the molecules of the discharge gas are excited or ionized to produce excited atoms, cations and electrons. The cations produced by ionization travel toward the cathode, and some of the cations collide with the cathode to produce secondary electrons. A plasma discharge is initiated by the synergistic effect of the ionization of the discharge gas by the electrons and the discharge of the secondary electrons by the cations. Note that the surface of the cathode contributing to the secondary electron emission will be referred to as a xe2x80x9ccathode layerxe2x80x9d, and the rest of the cathode excluding the xe2x80x9ccathode layerxe2x80x9d will be referred to as a xe2x80x9clower cathode layerxe2x80x9d.
While nickel is often used in the prior art as a material of the cathode layer, nickel is easily sputtered during a plasma discharge due to a high sputtering rate (the number of atoms sprung out of the cathode material when a single ion of the discharge gas collides therewith) of nickel, thereby causing the following two problems. One is the sputtered nickel atoms being attached to the plasma cell substrate and/or the dielectric layer bottom surface, thereby reducing the transmittance, and the other is the conductive nickel atoms being attached to the dielectric layer bottom surface along the cathode layer extending in parallel to the direction in which the plasma channels extend, thereby causing a phenomenon called xe2x80x9cbusbar phenomenonxe2x80x9d.
The busbar phenomenon will now be described. For example, a case where a color display is produced by using three contiguous picture element regions (respectively corresponding to red (R), green (G) and blue (B)) along a single plasma channel will be described. When only the center, green picture element region is turned ON (bright state), a predetermined amount of charge is induced in a region of the dielectric layer bottom surface corresponding to the green picture element region. However, if a conductive substance is attached to the dielectric layer bottom surface along the cathode layer, the induced charge diffuses in a direction along the cathode layer via the conductive substance to be distributed in regions of the dielectric layer bottom surface corresponding to the adjacent red and blue picture element regions beyond the region of the dielectric layer bottom surface corresponding to the green picture element region. Therefore, portions of the liquid crystal layer in the adjacent red and blue picture element regions change the orientation thereof by being influenced by the electric field (voltage) caused by the diffused charge. As a result, while only the green picture element region is supposed to be observed to be ON (bright state) with the adjacent red and blue picture element regions being OFF (dark state), portions of the red and blue picture element regions adjacent to the green picture element region, which is ON, are observed to be ON. Thus, a green color that is supposed to be displayed is mixed with a red color and a blue color, thereby reducing the color purity. As described above, a conductive substance attached to the dielectric layer bottom surface causes color mixture and reduces the display quality.
When nickel is used as the cathode layer, mercury is contained in the discharge gas in the prior art in order to ensure a sufficient product lifetime. While the mechanism by which mercury contributes to preventing the sputtering of the cathode layer has not yet been elucidated, it is presumed that a gas cloud of mercury covers the surface of the cathode layer, thereby absorbing the kinetic energy of discharge gas ions, and even if nickel is sputtered, the nickel atoms return to the surface of the cathode layer through collision with mercury atoms.
As described above, mercury contributes to preventing the sputtering of nickel. However, since the density of the mercury gas cloud depends upon the saturated vapor pressure, and the saturated vapor pressure has a logarithmic temperature dependency (according to the Rankine-Dupre""s formula), the sputtering preventing effect of mercury may not be expressed sufficiently in a low temperature region.
In view of this, the present applicant has proposed lanthanoid boride materials as a material of a cathode layer of a PALC (Japanese Patent Application No. 11-003543). For example, lanthanum hexaboride is used as a thermoelectron source of a scanning electron microscope, and is widely known as a substance having a good endurance. Gadolinium hexaboride, as lanthanum hexaboride, is a material having a good electron emission property since it has a small work function, and is suitable as a material of a cathode layer of a PALC. Since these materials have a smaller sputtering rate than nickel, the reduction of transmittance and the busbar phenomenon are less likely to occur even without filling a mercury gas, whereby it is possible to ensure a sufficient product lifetime even at low temperatures.
As the process of forming the cathode layer of a PALC, a sputtering method, an EB deposition method, an electrophoretic deposition method, and a printing method, are known, for example. These methods are generally classified into thin film formation processes such as the sputtering method and the EB deposition method, and thick film formation processes such as the electrophoretic deposition method and the printing method, and the thick film formation processes are used for improving the productivity and/or reducing the cost. In the electrophoretic deposition method or the printing method, a precursor cathode layer is first formed by using a mixture of a conductive material and an insulative material, and then the precursor cathode layer is baked at a temperature higher than the softening point of a binder material included in the insulative material to form the cathode layer. Typically, as the binder material, a glass, particularly a lead glass is used in many cases in order to reduce the process temperature. Lead in the lead glass is added in order to reduce the softening point thereof.
However, the present inventors have discovered that a sufficient product lifetime cannot be ensured if a lead glass, or the like, is used as the binder material as in the prior art, even in cases where lanthanoid boride materials having a high sputtering resistance are used as the material of the cathode layer.
For example, when a lead glass is used, a lead oxide included in the lead glass has a low sputtering resistance, and is sputtered during a plasma discharge to be attached to the plasma cell substrate and/or the dielectric layer bottom surface, thereby causing a reduction of the transmittance. Moreover, since a lead oxide is readily reducible, the lead oxide attached to the dielectric layer bottom surface is easily reduced to increase the conductivity, thereby causing the busbar phenomenon.
The above-described problem is common to gas discharge display devices having discharge electrodes, and plasma display panels (PDPS) producing a display by illuminating a fluorescent layer through a plasma discharge, as well as PALCs, have the problem that a sufficient product lifetime is not ensured. In a PDP, a lead oxide included in the lead glass in the cathode layer is sputtered during a plasma discharge to be attached to a front side substrate (e.g., a glass substrate) and/or the surface of the fluorescent layer, thereby reducing the transmittance and/or the illumination efficiency of the fluorescent layer, and thus reducing the illumination brightness.
The present invention has been made in view of the problems described above, and has an object to provide a gas discharge display device, a plasma addressed liquid crystal display device, and a method for producing the same, in which the reduction of the display quality due to sputtering of the cathode layer is prevented/suppressed.
A gas discharge display device of the present invention includes a pair of substrates opposing each other, and a plurality of plasma channels provided between the pair of substrates, wherein: each of the plurality of plasma channels includes a discharge gas, an anode and a cathode; and the cathode includes a cathode layer including a conductive material and a glass having a lead weight percentage of 30% or less, thus achieving the above-described object.
It is preferred that the glass includes at least one element selected from the group consisting of sodium, lithium, potassium and bismuth.
It is preferred that the conductive material includes gadolinium hexaboride, lanthanum hexaboride, yttrium tetraboride or carbon.
The gas discharge display device may further include an additional substrate opposing one of the pair of substrates via the other one of the pair of substrates, and a liquid crystal layer provided between the other one of the pair of substrates and the additional substrate.
Each of the plasma channels may further include a fluorescent layer.
An plasma addressed liquid crystal display device of the present invention includes a first substrate, a second substrate, a dielectric layer provided between the first substrate and the second substrate, a liquid crystal layer provided between the first substrate and the dielectric layer, and a plurality of plasma channels provided between the dielectric layer and the second substrate, wherein: each of the plasma channels includes a discharge gas, an anode and a cathode; and the cathode includes a cathode layer made of a mixture of a conductive material and an insulative material including a glass having a lead weight percentage of 30% or less, thus achieving the above-described object.
Another plasma addressed liquid crystal display device of the present invention includes a first substrate, a second substrate, a dielectric layer provided between the first substrate and the second substrate, a liquid crystal layer provided between the first substrate and the dielectric layer, and a plurality of plasma channels provided between the dielectric layer and the second substrate, wherein each of the plasma channels includes a discharge gas, an anode and a cathode, the plasma addressed liquid crystal display device being produced by a method for producing a plasma addressed liquid crystal display device, the method including the steps of: providing a second substrate; forming a precursor cathode layer on the second substrate by using a mixture of a conductive material and an insulative material including a glass having a lead weight percentage of 30% or less; forming a cathode including a cathode layer obtained by baking the precursor cathode layer; forming an anode on the second substrate, the anode opposing the cathode at a predetermined interval; attaching a dielectric layer to the second substrate at a predetermined interval, and then filling a discharge gas into a gap between the second substrate and the dielectric layer, thereby forming a plurality of plasma channels; and attaching the first substrate and the dielectric layer to each other at a predetermined interval, and then injecting a liquid crystal material into a gap between the first substrate and the dielectric layer, thereby forming a liquid crystal layer, thus achieving the above-described object.
It is preferred that the glass of the insulative material includes at least one element selected from the group consisting of sodium, lithium, potassium and bismuth.
It is preferred that the conductive material includes gadolinium hexaboride, lanthanum hexaboride, yttrium tetraboride or carbon.
A method for producing a plasma addressed liquid crystal display device of the present invention is a method for producing a plasma addressed liquid crystal display device including a first substrate, a second substrate, a dielectric layer provided between the first substrate and the second substrate, a liquid crystal layer provided between the first substrate and the dielectric layer, and a plurality of plasma channels provided between the dielectric layer and the second substrate, wherein each of the plasma channels includes a discharge gas, an anode and a cathode, the method including the steps of: forming a precursor cathode layer on the second substrate by using a mixture of a conductive material and an insulative material including a glass having a lead weight percentage of 30% or less; and forming the cathode including a cathode layer obtained by baking the precursor cathode layer, thus achieving the above-described object.
It is preferred that the step of forming the precursor cathode layer is performed by using an electrophoretic deposition method.
It is preferred that the step of forming the precursor cathode layer is performed by using a printing method.
It is preferred that the glass of the insulative material includes at least one element selected from the group consisting of sodium, lithium, potassium and bismuth.
It is preferred that the conductive material includes gadolinium hexaboride, lanthanum hexaboride, yttrium tetraboride or carbon.
Functions of the present invention will now be described.
In the gas discharge display device of the present invention, the glass included in the cathode layer is a glass having a lead weight percentage (mass percentage) of 30% or less. Therefore, the amount of a lead oxide to be sputtered during a plasma discharge is reduced. As a result, the reduction of the display quality is prevented/suppressed.
Also in the plasma addressed liquid crystal display device of the present invention, the glass included in the cathode layer is a glass having a lead weight percentage (mass percentage) of 30% or less. Therefore, the amount of a lead oxide to be sputtered during a plasma discharge to be attached to the second substrate and/or the dielectric layer bottom surface is reduced. As a result, the reduction of the transmittance and the occurrence of the busbar phenomenon are suppressed.
In the method for producing a plasma addressed liquid crystal display device of the present invention, an insulative material including a glass having a lead weight percentage of 30% or less is used in the step of forming the precursor cathode layer on the second substrate. Therefore, it is possible to obtain a plasma addressed liquid crystal display device having a cathode layer in which the content of a lead oxide, having a low sputtering resistance, is reduced. As a result, the reduction of the transmittance and the occurrence of the busbar phenomenon can be suppressed.
Where a step of forming a precursor cathode layer by using an electrophoretic deposition method is employed, the precursor cathode layer is formed on the surface of the lower cathode layer by immersing the second substrate having the lower cathode layer formed thereon in a solution (electrophoretic deposition solution) having a conductive material and an insulative material being dispersed therein, and by applying a voltage to the lower cathode layer. As a result, it is possible to improve the productivity and reduce the cost as compared to when a thin film formation process such as a sputtering method or an EB deposition method is used.
Where a step of forming a precursor cathode layer by using a printing method is employed, the precursor cathode layer is formed by printing a thick film paste including a conductive material and an insulative material on the second substrate. As a result, it is possible to improve the productivity and reduce the cost as compared to when a thin film formation process such as a sputtering method or an EB deposition method is used.
The softening point of a glass is reduced by including sodium, lithium, potassium or bismuth as an oxide. By adding a component listed above in place of lead, which is included in the prior art in order to reduce the softening point, it is possible to obtain a glass having a small lead content while minimizing the increase in the softening point from that of a conventional glass.
When gadolinium hexaboride, lanthanum hexaboride, yttrium tetraboride or carbon is used as the conductive material, the cathode layer is less likely to be sputtered during a plasma discharge because the materials listed above have high sputtering resistances. As a result, it is no longer necessary to fill in mercury where nickel is used as the cathode layer, as in the prior art, whereby it is possible to suppress the reduction of transmittance or the occurrence of the busbar phenomenon even at low temperatures.